Those Steam Keys are generated by Steam itself.
Only a retail seller(like g2a, humble bundle) will have a way to generate those keys, we can only have gifts in our inventory accounts.
So am I need to be a confirmed seller for Steam to buy and sell these keys? How can I be one of them? Is there any partnership page on steam?
Excellent question. I have no idea how to say that you want to be a retailer. Maybe try their contact e-mail?